The use of diesel engines in motor vehicles has greatly added to the atmospheric presence of harmful pollutants such as nitrogen oxides (NOx) and particulate matter (PM). Conventional diesel engines emit NOx and/or PM substantially in excess of desired environmental levels. Nevertheless, because of their fuel efficiency, diesel engines remain preferable to gasoline engines for many applications. Attempts to reduce NOx and PM emissions from diesel engines have therefore continued for many years.
Thus far, the prior art has not provided a robust diesel combustion system (i.e. providing commercially acceptable responsiveness and horsepower across diverse speed and load ranges) that is capable of maintaining engine-out emissions levels of both NOx and PM simultaneously within environmentally desired levels. The challenge of trying to maintain diesel engine emission levels of both NOx and PM simultaneously below environmentally acceptable levels has been a long-standing environmental and industry problem that has never fully been overcome despite extensive, concerted efforts by government and industry worldwide.
With the problem of simultaneously satisfactory engine-out NOx and PM reductions unsolved by the prior art for diesel engines, the diesel industry has instead turned primarily to development of NOx and PM aftertreatments (i.e. post-engine, but before the exhaust gas is released to the atmosphere) to meet current and upcoming international PM and NOx environmental regulations. However, such aftertreatment systems can be expensive, create size (“packaging”) concerns, and/or retain issues of effectiveness and durability. As an example, an ongoing challenge for NOx traps is maintaining NOx emissions always below desired levels on vehicles with diverse duty cycles, and the effectiveness and durability of NOx absorbers may also be undermined by factors such as high temperatures or by sulfate adsorption and desulfization due to sulfur content in diesel fuel. As a second example, to effectively maintain PM levels within environmentally acceptable emission levels, PM filter traps depend on engine-out PM emissions not being too high. In addition, PM filter traps must be regenerated (such as by combusting the particulate matter trapped therein), with the frequency of such renewal dependent upon the amount of engine-out PM the trap is forced to catch and retain.
Furthermore, global demand for diesel fuel is placing enormous pressure on world petroleum supplies. Competition for middle distillates has accelerated over the past decade due to rapid growth in demand in both Europe and the developing world, most notably in Asia. Indeed, the world economy and the environment rely heavily on ready supplies of low-sulfur diesel fuel. As conventional supplies of petroleum continue to tighten and eventually decline, finding sustainable alternatives to clean diesel fuel may also become a critical international priority.
Non-petroleum-derived diesel fuel alternatives include biodiesel from plant or vegetable oils, and synthetic diesel or DME from the gasification and reforming of natural gas, coal or biomass. In the United States, vast coal reserves and the availability of arable land make coal and biomass prime candidates as alternative fuel feedstocks.
Among the diesel fuel alternatives in the US, biodiesel and, to a somewhat lesser extent thus far, synthetic diesel have been used somewhat transparently as replacements for low-sulfur diesel fuel. DME has not yet seen commercial success in the US, due mainly to a lack of supporting infrastructure and the technical challenges of onboard fuel storage and fuel system reliability. Compared to biodiesel and synthetic diesel, DME has the advantage that its combustion generally produces very little PM, which potentially allows it to be used to meet US heavy-duty PM emissions requirements without the added cost of a PM filter trap. However, combustion of DME has traditionally produced high NOx emissions, and thus commercialization of a DME engine would be expected to require NOx aftertreatment to meet environmentally desired levels.
Most prototype DME injection systems used in engine demonstrations have been limited to peak injection pressures of between 250 and 300 bar. Little effort has been made to increase fuel injection pressure in DME engines, mainly because of the challenges of pumping a highly compressible, low-viscosity fuel efficiently. In addition, since DME is somewhat like propane in that it flashes to a gas at atmospheric pressure, it may have been felt that there would be little or no advantage to, higher pressure injection of DME.